An optical disc is a flat, usually circular disc that encodes binary data (bits). One form of optical recording has pits representing a binary value of 0 or “off,” due to lack of reflection when read, and lands representing a binary value of 1 or “on,” due to a reflection when read. The pits and lands are covered with a special reflective material. In another form of optical recording, the transition between pit and land represents the binary value of 1 and the intervening length of pit or land between the pit-land transitions represents one or more successive zeros. For read-only-memory “ROM” optical disks, the depth of the stamped pit is typically ¼ of the wavelength of the laser reading the data, and the ensuing cancellation of the light reflected off the bottom of the pit with the incoming light makes the pit appear dark compared to the surrounding land. For “ROM” optical disks, the lands and stamped pits are covered by a reflective material such as aluminum or gold, and the laser can read through one data layer of the optical disk to subsequent inner data layers. For rewritable “RW” optical disks, the pit does not have a change in depth from the surrounding land, but it is actually a second-phase of a phase-change recording layer, and this phase appears darker than the surrounding first-phase of this same phase-change recording layer. An optical dye can be used as the recording material in write-once, read-many “WORM” optical disks, where the laser essentially creates a permanent mark on the disk, herein called a pit. “ROM,” rewriteable “RW,” and “WORM” optical disks may have the same recording format, as is the case for Digital Versatile Disks (DVD) and BLU-RAY Disks (BD) so that DVD disks can be read by the same DVD player, and BD disks can be read by the same BD player. The encoding material sits atop a thicker substrate (usually polycarbonate) that makes up the bulk of the disc and forms a dust-defocusing layer. The encoding pattern follows a continuous, spiral path which encompasses the disc's data-surface and extending from the innermost track to the outermost track. The data is stored on the disc with a laser or stamping machine, and can be accessed when the data path is illuminated with a laser diode in an optical disc drive which spins the disc at speeds of about 200 to 4000 RPM (revolutions per minute) or more, depending on the drive type, disc format, and the distance of the read head from the center-of-rotation of the disc (inner tracks are read at a faster disc RPM for constant-linear-velocity “CLV” discs). Typical layer wavelengths are 405 nm for a blue-laser (typical for BD), 650 nm for red-laser (typical for DVD), and 780 nm for infrared laser (typical for CD or compact disks). The shorter the laser wavelength, the more narrow the width of the pits, which allows for more revolution of the spiral track of data, hence the higher the data capacity. The pits, which may be called bumps if the disks are observed upside-down, and the spiral track itself, distort ambient light, hence most optical discs characteristically have an iridescent appearance to the human eye, one created by the groove of the spiral track and the reflective layer. The reverse side of an optical disc usually has a printed label, sometimes made of paper but often printed or stamped onto the disc itself. This side of the disc contains the actual data and is typically coated with a transparent material, usually lacquer. Unlike the 3½-inch floppy disc, most optical discs do not have an integrated protective casing and are therefore susceptible to data transfer problems due to scratches, fingerprints, and other environmental problems.
Optical discs are usually between 7.6 and 30 cm (3 to 12 in) in diameter, with 12 cm (4.75 in) being the most common size. A typical disc is about 1.2 mm (0.05 in) thick, while the track pitch (distance from the center of one track to the center of the next) is typically 1.6 μm.
An optical disc is designed to support one of three recording types: read-only (e.g.: CD, CD-ROM, DVD, DVD-ROM), recordable (write-once, e.g. CD-R, DVD-R, BD-R), or re-recordable (rewritable, e.g. CD-RW, DVD-RW, and BD-RW). Write-once optical discs commonly have an organic-dye recording layer between the substrate and the reflective layer. Rewritable discs typically contain an alloy recording layer composed of a phase-change material, most often AgInSbTe, an alloy of silver, indium, antimony, and tellurium.
Optical discs include CDs, DVDs, and BLU-RAY. Optical discs are most commonly used for storing music (e.g. for use in a CD player), video (e.g. for use in a BLU-RAY player), or data and programs for personal computers (PC). The Optical Storage Technology Association (OSTA) promotes standardized optical storage formats. Although optical discs are more durable than earlier audio-visual and data storage formats, they are susceptible to environmental and daily-use damage. Libraries and archives enact optical media preservation procedures to ensure continued usability in the computer's optical disc drive or corresponding disc player.
Optical disc offers a number of advantages over magnetic storage media. An optical disc holds much more data. The greater control and focus possible with laser beams (in comparison to tiny magnetic heads) means that more data can be written into a smaller space. Storage capacity increases with each new generation of optical media. Emerging standards, such as BLU-RAY, offer up to 27 gigabytes (GB) on a single-sided 12-centimeter disc. In comparison, a 3.5″ floppy diskette, for example, can hold 1.44 megabytes (MB). Thus, one BLU-RAY disk can hold the equivalent of 18,750 3.5″ floppy diskettes. Optical discs are inexpensive to manufacture and data stored on them is relatively impervious to most environmental threats, such as power surges, or magnetic disturbances.
Optical discs may have a single-sided or dual-sided configuration. Each side may have a single or multiple layers of optical media. Currently single or dual layers are common. However, multiple layers such as 3, 4 or greater may be formed in the future, particularly for the BLU-RAY disk as the depth of the BD recording layer within the surrounding substrate is less than for DVD. DVD kept the CD depth of the recording layer, for commonality between DVD and CD disks, but this greater recording depth within the surrounding substrate caused more optical-distortion for higher densities. Thus BD disks, with their more-shallow depth of recording layer, afford the greatest potential for increases in data layers beyond two.